1. Technical Field
The present invention relates to a signal generating device that generates a timing pulse signal determining a liquid ejection timing in a liquid ejecting apparatus, such as an ink jet printer or the like, and to a liquid ejecting apparatus incorporating such a signal generating device.
2. Background Art
A printer has an encoder that is provided to detect a position of a movable body, such as a carriage or the like, and the position of the movable body is detected by counting the number of pulses of a pulse signal to be output from the encoder. Further, during the movement of the movable body, such as a carriage or the like, an ink droplet is ejected from a printing head mounted on the carriage and then is landed at a target position on a recording medium, such as paper or the like, so that printing is performed.
The carriage reciprocates in a primary scanning direction, and there is a flight time period until the ink droplets ejected from the printing head are landed on the paper. Accordingly, during the flight time period, the ink droplets move in a moving direction of the carriage (that is, the moving direction of the printing head). For this reason, even when the carriage reaches the same position upon both the forward movement and the backward movement, actual landing positions of the ejected ink droplets are deviated along the moving direction of the carriage, and the actual landing position and a target landing position are deviated from each other. The actual landing position is deviated from the target landing position, that is, a so-called bi-directional deviation occurs. For this reason, an ejection timing of the ink droplets from the printing head is controlled so that the bi-directional deviation can be prevented so as to allow the ink droplets to be accurately landed at the target landing position.
Japanese Patent Publication No. 9-136465A (JP-A-9-136465) discloses a signal generating device that generates a plurality of signals by dividing an encoder signal for high-resolution printing. In this device, a pulse width of the encoder signal is measured (counted), and the count value is divided. Then, the divided value is counted down by a down-counter, and a BORROW signal for performing a shift is counted by an internal pulse counter. The internal pulse counter creates 16 internal pulse signals by counting 16 BORROW signals and generates an output timing of each internal pulse signal for each count value set in an output pulse controller. Therefore, a printing signal that has a cycle different from that of the encoder signal is output. With this configuration, even though the movement velocity of the movable body is varied, it is possible to generate the internal timing signal so as to correspond to the movable body.
Further, the ejection of the ink droplets from the printing head is normally performed in a constant-velocity region of the carriage. For example, Japanese Patent Publication No. 2004-50771A (JP-A-2004-50771) discloses a printer that widens a printing region by ejecting ink droplets in an accelerating region of the carriage.
However, the signal generating device disclosed in JP-A-9-136465 divides the pulse width of the previous encoder signal (1/n) and generates an internal timing signal. In this case, the constant-velocity region or a velocity changing region where a change in pulse width of the previous encoder signal and a change in pulse width of the current encoder signal are small is assumed. Specifically, the known signal generating device generates the internal timing signal by dividing the measured pulse width of the encoder signal into 16 segments. Accordingly, when a change between a previous encoder cycle and a current encoder cycle is 1/16 (6.25%) or more, it is difficult to generate the prescribed number of internal timing signals.
FIG. 15A is a timing chart when an internal timing signal in an accelerating region of a movable body is generated in the same manner as that in JP-A-9-136465. As shown in FIG. 15A, a reference pulse of a rising edge detector is obtained from the encoder signal, and a count value for one cycle (cyclic count value) is obtained by counting a cycle of the reference pulse using an up-counter. Then, a BORROW signal to be output when a value (for example, “4”) obtained by dividing the previous cyclic count value (a value regarded as a current cyclic count value) by a prescribed number (for example, “8”) for division of one cycle (cyclic count value: for example, “32”) is counted down by a down-counter is counted by an internal pulse counter and a pulse is generated for each count. Accordingly, an internal timing signal having a plurality (for example, “16”) of pulses is generated per a cycle of the reference cycle.
As shown in FIG. 15A, upon rapid acceleration or deceleration for performing high speed printing, the change between the encoder cycles TA and TB becomes large, the internal timing signal generated each time the value (in this example, CA/n=4) obtained by dividing the cycle TA (cyclic count value CA) by a prescribed number n is counted down cannot be output for the next one cycle TB by a prescribed number (for example, seven). That is, for the cycle TB, seven internal timing signals have to be generated, but only five internal timing signals can be generated, as shown in FIG. 11A. In addition, at the next cycle TC, similarly, since the change between the encoder cycles TB and TC is large, only five internal timing signals, not seven, are generated.
In addition, as shown in FIG. 15B, even though the prescribed number (eight) of internal timing signals can be generated, a time interval tx between the eighth output and the next first output is made extremely short compared with other time intervals (cycle TB). In such a circuit configuration, there is a problem in that a correct print timing signal cannot be obtained in the accelerating region. Further, in order to obtain a higher-resolution image, when the number of output internal timing signals to be generated per a cycle of the reference cycle increases by increasing the number of divisions of the reference pulse, the above problem more drastically occurs.
Also Japanese Patent Publication No. 11-334146A (JP-A-11-334146) discloses a background art of the invention.